The present invention relates generally to alternators, and more specifically to a rectifier reconfiguration scheme for an alternator.
Alternators are used in automotive vehicles to generate electricity that may be used to power the electrical components of the vehicle and to charge the vehicle battery. The alternator is typically utilized to provide a direct current (DC) output. A conventional motor vehicle alternator includes a three-phase stator winding with a rotor magnetically coupled to the stator. The rotor is driven by the vehicle""s internal combustion engine. As the rotor is driven, three-phase current generated at the output connections of the stator windings is rectified by a three-phase diode rectifier. The output current and efficiency of the alternator vary with the speed of the engine which is controlled in accordance with the operational requirements of the vehicle.
One key operation parameter is the ratio between the phase voltage and the number of turns of the phase windings. Unfortunately, the optimum voltage to turn ratio that yields maximum output is not constant. The optimum voltage to turn ratio increases proportionally as the generator speed increases. Therefore, if the voltage to turn ratio of the generator can be increased as the generator speed increases, the generator output and efficiency will be increased at high generator speeds.
One method to increase the voltage to turns ratio is to reduce the number of turns while keeping the voltage the same. One such system is disclosed in commonly assigned U.S. Pat. No. 5,642,021 to the present inventors. Another possible approach is to change the rectifier configuration from a full-wave rectifier to a half-wave rectifier at high speed. By changing the rectifier configuration from full-wave to half-wave, the phase voltage can be increased as a result. The voltage to turn ratio is thus closer to an optimum value when the alternator is at high speed. However, the alternator may lose balanced operation after the rectifier is changed from full-wave to a half-wave configuration because the half-wave rectifier only allows one phase winding to conduct at any moment of time. Unbalanced operation leads to alternator vibration and higher noise level.
It is particularly important in the operation of a hybrid electric vehicle to provide a system having high power density (output per unit volume) and efficiency.
One object of the present invention is to provide an alternator system having a high power density and efficiency.
In one aspect of the invention, a rectifier circuit or an alternator has a switch circuit having a first side and a second side. The rectifier circuit has a first half-wave diode rectifier coupled to the first side and a second half-wave diode rectifier coupled to the second side. A first three-phase stator winding is coupled to the first half-wave diode rectifier and a second three-phase stator winding is coupled to the second half-wave diode rectifier. A controller is coupled to the switch circuit for controlling the switch circuit from an open state to a closed state so that the rectifier configuration can be changed.
In a further aspect of the invention, a method for controlling an alternator having a first three-phase winding coupled to a first half-wave rectifier and a second three-phase winding coupled to a second half-wave diode rectifier comprises the steps of:
generating a speed signal corresponding to the rotational speed of the alternator;
coupling the first half-wave rectifier and the second half-wave rectifier together when the speed of the alternator is below a threshold to form a full-wave rectifier; and
forming two parallel half-wave rectifiers when the speed signal is above a predetermined threshold.
One advantage of the invention is that when providing two parallel half-wave rectifiers, the operation of the alternator is balanced. By providing balanced operation of the alternator, alternator vibration and thus the noise associated therewith is reduced. Another advantage of the invention is that the efficiency of the alternator is increased at high speeds.